1. Field of the Invention
The present invention relates to an optical element, such as a polarized-light-separating element, a color filter, and a retardation film, for use in a liquid crystal display and the like. More particularly, the present invention relates to an optical element including a liquid crystal layer made from a liquid crystalline material such as a cholesteric liquid crystal and a nematic liquid crystal. The term “liquid crystal layer” as used herein refers to a layer having the nature of liquid crystal in an optical sense, and encompasses a layer in the solidified state that a liquid crystal phase is solidified with its molecular orientation maintained as it is.
2. Description of Related Art
In general, a conventional liquid crystal display is composed of: a liquid crystal cell that changes, in each pixel, the state of polarization of illumination light emitted by an illuminator (light source); a pair of polarizers that are arranged on the illumination light incident and emergent sides of the liquid crystal cell so that the liquid crystal cell is sandwiched between them; color filters (red, green and blue) that are provided on the individual pixels of the liquid crystal cell; and a retardation film (optical compensation sheet) for improving the visibility of the liquid crystal cell.
In the above-described conventional liquid crystal display, the illumination light emitted by the illuminator is usually non-polarized light, and when this light passes through the polarizer arranged on the illumination light incident side of the liquid crystal cell, 50% or more of the light is absorbed by the polarizer. Further, the illumination light emitted by the illuminator is usually white light, and when this light passes through the color filters (red, green and blue) provided on the individual pixels of the liquid crystal cell, 70% or more of the light is absorbed by the color filters. Namely, in a conventional liquid crystal display, illumination light emitted by an illuminator is mostly absorbed before it goes out from the viewing side of the display. Thus, the optical efficiency of a conventional liquid crystal display has been not necessarily high.
An illuminator with high output is therefore required for a conventional liquid crystal display of the above-described type to attain a display with sufficiently high brightness. However, the use of such an illuminator leads to a great increase in power consumption.
Under these circumstances, there has been proposed a method for efficiently utilizing light such as illumination light by the use of an optical element comprising a liquid crystal layer, such as a polarized-light-separating element or a color filter, wherein the optical element selectively transmits a part of light while reflecting a part of the reminder, and the reflected light is reused with the aid of a reflector or the like. Specifically, Japanese Patent Publication No. 2,509,372, for example, proposes a method for efficiently separating illumination light (non-polarized light) emitted by an illuminator to obtain specific polarized light. In this method, a polarized-light-separating element including a cholesteric liquid crystal layer is used in combination with a reflector capable of reflecting, while inverting the direction of rotation of light (circularly polarized light) that has been reflected by the polarized-light-separating element.
In addition, an attempt has been proposed in which a retardation film for eliminating a viewing-angle dependency is realized by utilizing a nematic liquid crystal layer having nematic regularity or a cholesteric liquid crystal layer having cholesteric regularity. Not only a λ/4 retardation film of a general band, but also a λ/4 retardation film of a broadband have been proposed for such a retardation film.
However, the part of the above-described optical element that functions optically is a liquid crystal layer made from a liquid crystalline material, so that this part usually has extremely low hardness even when the liquid crystal layer is in the solid phase state. Therefore, if forces are externally exerted to the liquid crystal layer in the process of production of the optical element or in the course of incorporation of the optical element in a liquid crystal display, there is the possibility that the liquid crystal layer has dents or the like on its surface and cannot maintain its uniform film thickness distribution. In the case where the liquid crystal layer in the above-described optical element has no uniform film thickness distribution, the state of polarization of light that comes out from the optical element is not uniform. If such an optical element is incorporated in a liquid crystal display, it is inevitable that the liquid crystal display has considerably lowered displaying quality. Further, if the above-described optical element is used as a retardation film to be incorporated in a liquid crystal cell of a liquid crystal display, a film thickness of the liquid crystal layer is varied by a spacer for holding a gap in the liquid crystal cell. As a result, there is generated a problem in that a desired retardation amount cannot be obtained.